What is Ferroresonance and Application Area
1. INTRODUCTION
Ferroresonance is a nonlinear resonance phenomenon that affects power networks. The term “Ferroresonance” was used for the first time in 1920 for express the oscillating phenomena in an electric circuit. Circuit must contain at least below circuit elements as it can be defined as ferroresonance:
- Non -linear saturable inductance (ex; transformer)
- Capacitor
- Resistor
In a circuit consists of these elements; by the changing the values of an electrical component, the values of current and voltage values on the terminals at other components change suddenly.
Ferroresonance begins some switching events such as; load rejection, energizing or de-energizing of transformer, switching of circuit breaker. The causes risk to it can be listed as follows:
- Idling or operation on low load on the systems grounded neutral point
- Insulation faults,
- Overloading or idling of voltage transformer
- Switching one or two phases
- Dissymmetry caused by connection errors
- Switching transformers or capacitors
- Connecting low load power transformers to network with short circuit load
- Long and / or capacitive cable feeding a transformer
- Streak of lightning on transmission lines
When the ferroresonance occurs some of the below symptoms are accompanied:
- Phase-to-phase or phase-to-neutral long – term overvoltages
- Long – term overcurrents
- Distortion on the current and voltage waveforms
- Displacement of the neutral point voltage
- Transformer overheating (at no load)
- Load noise in voltage transformers and overheating in core and primary windings
- The failures cause of thermal effect or insulation breakdown on electrical materials (capacitor banks, current – voltage transformers etc.)
2. FERRORESONANCE ON THE VOLTAGE TRANSFORMERS
A good example of VT destruction by ferroresonance is destroyed primary winding and intact secondary winding.
However, all of these symptoms are not specific to the ferroresonance. For example; displacement of the neutral point may be the consequence of a single phase to earth fault an unearthed neutral system.
Voltage transformers can be into two different types as inductive and capacitive. Inductive voltage transformers are more prone to ferroresonance. Because, they have more inductive characters and so they need more capacitance to convert ferroresonance circuit.
In ferromagnetic circuits which wirings of ferromagnetic materials such as iron, ferroresonance occurs because of the inductance. Transformers are an excellent example of ferromagnetic inductance.
Inductance of the voltage transformers and grounding capacitance of the cables generate a RLC circuit oscillation potentially.
The equality between inductive reactance (X_L) and capacitive reactance (X_C) leads to resonance.
- W L = 1 / W C
In a resonance RLC circuit, low frequency oscillations (16,7 Hz for 50 Hz system) starts when the over voltage ended. This low frequency oscillations cause to saturation on voltage transformers’ magnetic windings because magnetic reactance decrease.
The current density which circulating on primary windings of voltage transformers cause to overheating and consequently produce overpressure.
The reactive energy in an oscillation circuit between two components and powered continuously by network is converted to heat in voltage transformer until it burn. Consequently, secondary windings damage while primary windings are good.
Picture 1-2: Primary windings of voltage transformer exposed by ferroresonance
Picture 3-4: Secondary windings of voltage transformer exposed by ferroresonance
3. PREVENTING FERRORESONANCE ON VOLTAGE TRANSFORMERS
When the single pole inductive voltage transformer is used, keep in mind that ferroresonance can be occur if the circuit close or in the process of earthing fault damping.
Ferroresonance may cause to overheating of voltage transformers and accordingly damage or over induction. It can be damped only by lowering the voltage or connecting a fixed ohmic resistance.
Although predicting when the ferroresonance might occur, the risk of it can be preventing or reducing by taking precautions in advance.
There are some preventive measures for ferroresonance. However, the most practical and the most economical method is using ohmic resistance with open-delta winding on voltage transformers’ secondary wirings.
When the voltage transformers’ protection windings are used as circuit in Figure-1 (a fixed ohmic resistor is connected to ends of open-delta connection) third harmonic currents flows and thus the resonance is prevented. The advantages of this resistor; it doesn’t affect measurement precision and doesn’t cause to any loss under normal operation conditions. Open-delta circuit should be grounded from only one point shown as Figure-1. Resistor damps only unbalanced situations. On balanced situations, there is no current flow at open- delta circuit.
4. CALCULATION OF THE DAMPING RESISTOR
The magnitude of the damping resistor can be calculated with the values on the label of voltage transformer. Calculations of the minimum resistance (R) and power (P_R) values for the damping resistor:
` R_D >= (U_\Delta^2)\overS_\Delta = sqrt3∙(U_{sr}^2)/S_{sr} P_v >= (3,3∙U_{sr})^2/R_D `
U sr : Rated voltage of VT’s secondary winding that connected resistor
S v : Rated thermal load of VT’s secondary winding related with resistor (VA)
5. CONCLUSION
At energy transmission systems, as the number of wiring systems increase, protection of voltage transformers has become very important in order that the systems work continuously and levelly.
Ferroresonance at power systems can be too harmful for electrical equipment because it’s stable overcurrent and overvoltage characteristic.
Therefore, systems should be designed by taking into consideration to ferroresonance risk. Because it is an effective method for preventing on ferroresonance, open-delta connection and damping resistor should be used on all voltage transformers
6. REFERENCES
[1] D.A.N. Jacobson, “Examples of Ferroresonance in a High Voltagepower System”, in Power Engineering Society.
[2] F.Akça, “Enerji İletim Sistemlerinde Ferrorezonans Olayları”, SAU Fen BilimleriEnstitüsü Dergisi, 6.Cilt, 2. Sayı, 2002.
[3] V.Valverde, A.J. Mazon, I.Zamora, G.Buiges, “Ferroresonance in Voltage Transformers: Analysis and Simulations”, Electrical Engineering Department E.T.S.I.I.
[4] R.C. Dugan,” Examples of Ferroresonance in Distribution Systems”, Fellor, IEEE
[5] Ferracci, P.: “Ferroresonance”, Group Schneider: Cahier nº 190, 1998.
[6] R. F. Karlicek and E. R. Taylor, “Ferroresonance of Grounded Potential Transformers on Ungrounded Power Systems”, Power Apparatus and Systems, Part III. Transactions of the American Institute of Electrical Engineers, 1959.
[7] Erbay, Ali, ” Parameter Study of Ferro-Resonance with Harmonic Balance Method”, Degree Project in Electric Power Systems Second Level, 201
[8] Siemens, “Damping resistor “, 2005.
